1. Field of the Invention
The present invention relates to a laser oscillator of an electric discharge pumping type of a gas laser medium for use in laser machining, medical treatment, illumination and communication, etc. and particularly to a laser oscillator having function of controlling a lateral mode (hereinafter referred to as beam mode) of a laser output.
2. Description of Related Art
There is known a gas laser oscillator for use in laser machining, medical treatment, illumination and communication, etc. having one or more electric discharge sections with electrodes each connected to an electric-discharge excitation power source. FIG. 6 schematically shows a typical example of such gas laser oscillator. In FIG. 6, an optical resonating space is formed between a rear mirror 4a of a total reflection mirror and an output mirror 4b of a partial reflection mirror. Two electric discharge sections 3a, 3b are provided in the optical resonating space.
The medium gas circulates along circulating paths through the optical resonating space by a blower 6. The medium gas discharged from the blower 6 passes through a heat exchanger 5a for removing compression heat and is supplied to the electric discharge sections 3a, 3b. The electric discharge sections 3a, 3b have electrodes 2a, 2b connected to electric-discharge excitation power sources 1a, 1b, respectively, for pumping the gas medium by electric discharge between the electrodes 2a, 2b to generate a laser beam. The generated laser beam is amplified by the optical resonator and outputted from the output mirror 4b. The gas medium heated by the electric discharge is cooled by the heat exchanger 5b to return to the blower 6.
In this example, two electric discharge sections 3a, 3b each having an electric discharge tube are excited by the respective power sources 1a, 1b. The power sources 1a, 1b supply alternating current and thus the electric discharges generated in the electric discharge sections 3a, 3b are alternating electric discharges.
Generally, a beam mode of the gas laser oscillator of this type is determined in dependence on an arrangement and a size of the laser resonator. For example, the beam mode changes in accordance with a length of the laser resonator, a sectional shape and a size of the electric discharge section. Further, in the case where an electric discharge tube is adopted for constituting the electric discharge section, factors determining the beam mode include an inner diameter of an aperture provided on an optical path (not shown in FIG. 6) as well as an inner diameter of the electric discharge tube and shapes of the electrodes. These factors are discussed in detail in JP 64-42187, for example.
The beam mode of the gas laser oscillator should be set in accordance with a use of the laser oscillator in practice such as machining, and it is desirable to appropriately control the beam mode to have characteristic suitable for the use of the laser oscillator in practice so as to cope with various uses.
There is known a technique of controlling the beam mode using an aperture as disclosed in EP 0492340A. In this technique, an aperture for restricting a beam diameter is inserted and retracted in an optical path of a laser resonator so as to change the beam mode. Mode change is realized between a TEM00 mode (Gauss mode) or a low degree mode, and a TEM01* mode (ring mode) or a high degree mode by switching a state of the aperture inserted on an optical axis and a state of the aperture retracted from the optical path.
This method of mechanically actuating the aperture generally has a problem in durability and low adaptiveness to a high speed control of the beam mode in view of response characteristic. Also, adjustment of an optical axis of the aperture is difficult to have difficulty in handling and maintenance.
Another technique of controlling the beam mode is known from JP 2002-118312A. In this document, an adaptive mirror (curvature variable mirror) is used for controlling the beam mode and the mode change is performed between two set states of mechanically variable curvature of the mirror. However, in this method of changing the curvature of the mirror also has problems in response characteristic and controllability as far as a shape of the mirror is mechanically varied and fails in solving difficulty in adjustment of an optical axis of the mirror. The function and structure of the adaptive mirror are described in detail in JP 3072519B.